1. Field of the Invention
This invention relates to storage system caches and, more particularly, to metadata design within the storage system cache.
2. Description of the Related Art
Many storage systems employ non-volatile caching as a means of improving input/output (I/O) performance between the storage subsystem and the host(s). In addition, the non-volatile cache may be used to improve internal storage system operations such as error logging, recovery, and reconstruction for example.
Many such storage caches include data storage as well as metadata storage. Metadata typically includes information that describes the data. For example, metadata may include validity information and a pointer to the location of the data. Thus, at any point in time, the integrity of the metadata is critical. For systems having a high reliability, availability and serviceability (RAS) rating, in addition to the data, the metadata is generally stored in a storage medium that is non-volatile and which can survive from such catastrophic failures as system crashes and power failures.
Generally speaking, the storage cache may be implemented to be addressable in either the CPU memory space or in the I/O space. There are design implications for both approaches. For example, if the storage cache is placed in the CPU memory space, as the size of the storage cache increases, it may become impractical to address the entire cache in the CPU memory space. In addition, there may be limited memory bandwidth. Further, to place the storage cache in the CPU memory space may require a large part of the system memory to survive a power loss, thereby requiring a large battery backup. If the storage cache is placed in the I/O space, data requests may be markedly slow due to such conditions as slower I/O buses compared to the CPU memory bus and constrained accessibility of the I/O buses, for example. This may be particularly true for read operations into the I/O space. However, one positive attribute of having the storage cache placed in the I/O space is that the requirement for battery backup may be reduced.
In high availability systems, the system may employ cache mirroring. In such systems, a storage cache may be mirrored onto another cache system in an independent failure domain. Such systems typically employ a fixed structure, self-describing cache in the I/O space. The self-describing cache uses a portion of the cache space to store enough information to identify the dirty and the valid blocks in the storage cache. However, when the primary cache fails, there is a delay associated with reconstructing the metadata from the self-describing cache. This reconstruction delay impacts the availability and performance of the system under failure conditions. Therefore a key aspect of the storage cache design is the structure and management of the metadata.
In addition, in various systems, the cache may be used to store more than one copy of the user data that is stored in the underlying storage volume at a given time. For example, data having a related association with the data stored in the underlying storage volume may also be stored in the cache. One example of such related data is generational data. Another example of related data is the Exclusive OR or “Parity” data of current data and newly written data. One technique used for storing related data in the cache involves dis-aggregating or splitting the cache and using the different portions of the cache to store different types of data and their associated metadata. However, as more powerful metadata structures are employed, the metadata size may increase, thereby increasing the metadata overhead. In addition, splitting the cache may increase the cache management burden by having to manage more than one type of cache with respect to allocation, population and de-staging, for example.